CH11 DSP System Design ,Software-Defined Radio

CH11 DSP System Design ,Software-Defined Radio - CHAPTER 11...

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C H A P T E R 11 DSP System Design: Software-Defined Radio This chapter covers a software-defined radio system built by using the LabVIEW hybrid programming approach. A software-defined radio consists of a programmable communication system where functional changes can be made by merely updating software. For a detailed description of software-defined radio, the reader is referred to [ 1 ], [ 2 ]. 4-QAM (Quadrature Amplitude Modulation) is chosen to be the modulation scheme of our software-defined radio system, noting that this modulation is widely used for data transmission applications over bandpass channels such as FAX modem, high-speed cable, multi-tone wireless, and satellite systems [ 2 ]. For simplicity, here the communication channel is considered to be ideal or noise-free. 11.1 QAM Transmitter For transmission, pseudo noise (PN) sequences are generated to serve as our message signal. A PN sequence is generated with a five-stage linear feedback shift register structure, as shown in Figure 11-1 , whose connection polynomial is given by h ð D Þ ¼ 1 þ D 2 þ D 5 (11.1) where D denotes delay and the summations represent modulo 2 additions. The sequence generated by the preceding equation has a period of 31( ¼ 2 5 ± 1). Two PN sequence generators are used in order to create the message sequences for both the in-phase and quadrature phase components. The constellation of 4-QAM is shown in Figure 11-2 . For more details of PN sequence generation, refer to [ 2 ]. 277
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Note that frame marker bits are inserted in front of the generated PN sequences. This is done for frame synchronization purposes, as discussed in the following receiver section. As illustrated in Figure 11-3 , a total of 10 frame maker bits is located in front of each period of a PN sequence. Q 1 1 I 1 1 Figure 11-2: Constellation of 4-QAM. 1 0 Frame Marker Bits Frame Marker Bits PN Sequence (Length = 31) Amplitude 1 Figure 11-3: PN sequence generator. + + + + + D D D D D y ( n 1) h 1 h 2 h 3 h 4 h 5 y ( n 2) y ( n 3) y ( n 4) y ( n 5) y ( n ) x ( n ) Figure 11-1: PN generation with linear feedback shift register. 278 Chapter 11
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The generated message sequences are then passed through a raised-cosine FIR filter to create a band-limited baseband signal. The frequency response of the raised cosine filter is given by G ð f Þ ¼ 1 for j f j ² ð 1 ± a Þ f c cos 2 p 4 a f c ± j f j ± ð 1 ± a Þ f c ² " # for ð 1 ± a Þ f c ² j f j ² ð 1 þ a Þ f c 0 elsewhere 8 > > > > < > > > > : (11.2) where a 2 ½ 0 ; 1 denotes a roll-off factor specifying the excess bandwidth beyond the Nyquist frequency f c . The output of the raised cosine filter is then used to build a complex envelope, ~ s ð t Þ , of a QAM signal expressed by ~ s ð t Þ ¼ X 1 k ¼±1 c k g T ð t ± kT Þ (11.3) where c k indicates a complex message, made up of two real messages a k and b k , c k ¼ a k þ jb k .
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